This invention relates to a method of seismic exploration and more especially to a method which consists in sending into the medium to be explored wave trains made up of acoustic vibrational waves of long duration and having a frequency which is continuously variable between two limiting frequencies, in receiving the emitted signal in at least one receiver or seismograph after reflection from the different reflectors of the medium to be explored, then in processing the signals which have been received and recorded.
The complex amplitude spectrum of the emitted signal is never rectangular as might be expected but on the contrary irregular especially insofar as it extends beyond the limiting frequencies; the term complex is used in the mathematical sense and should accordingly be taken to mean that the spectrum can have real and/or imaginary portions. This phenomenon arises from the fact that the emitted signal is limited in time and therefore contains a finite number of arches.
The emitted signal follows a convolute path within the medium to be explored, with the reflectors which constitute the dioptric elements for the waves which travel within said medium; the received signal which corresponds to the emitted signal is therefore composed of a sum of different signals weighted by the coefficient of reflection of said reflectors, said received signal being displaced with respect to the double time intervals of the path between reflectors.
Processing of the received signals consists in correlating each received signal or recording by the signal which has given rise to this latter. This operation leads to a result equivalent to the convolution of the pulsed final seismogram by the autocorrelation function of the emitted signal. In order to permit of easy differentiation of very closely spaced reflectors, which is the object of the so-called high-resolution seismic exploration technique, the autocorrelation function of the emitted signal must have a central peak which is as narrow as possible.
Moreover, in order to ensure that the image of a reflector is as accurate as possible, said image must not be impaired by effects produced by other reflectors even in remote locations. The result of this is that the correlation residues or noise of the autocorrelation function must necessarily be as small as possible.
As a consequence of the foregoing, it is essential to take certain precautions at the time of emission on the amplitude spectra of the emitted signals: a high-resolution signal must have an amplitude spectrum which is more spread-out than a medium-resolution signal and a signal having low correlation noise must exhibit an amplitude spectrum having a higher degree of "smoothness" or "uniformity" than a signal having high correlation noise.
In the vibrational method, it is possible to modify the shape of the spectrum of frequencies emitted either at the time of emission or after reception at the time of the correlation operation or by means of any other suitable operation. Should it be desired to produce action on the shape of the spectrum of frequencies emitted after reception, however, the signal-to-noise ratio must be sufficiently good to ensure that a noise is not unduly amplified when the amplitudes of certain frequencies of the received signal are amplified.
One proposed improvement has consisted in dividing the spectrum of emitted frequencies into frequency bands and in successively or simultaneously emitting said frequency bands, then processing the corresponding received signals in the manner described in the foregoing. However, this method is still subject to many disadvantages. In fact, since the amplitude spectra of the emitted signals do not in actual practice have perfectly adjacent square-wave shapes but exhibit considerable differences with respect to the square-wave shape as already mentioned, the sum of the amplitude spectra of the signals corresponding to the different bands is not equal to the amplitude spectrum of the entire spectrum of emitted frequencies.
Moreover, when the autocorrelations of these elementary signals corresponding to the frequency bands are performed, it is found that the sum of the autocorrelations is very different from the autocorrelation of the origin signal and especially that the sum correlation has a substantial correlation residue arising from the fact that the amplitude spectra of the elementary signals overlap with respect to each other.
Thus, if the conventional method is employed to perform the correlation of each recorded line or received signal with the emitted signal and then the summation of the lines corresponding to the different frequency passbands, there will again be found a seismogram equivalent to the pulsed seismogram convoluted by an autocorrelation having a substantial correlation residue. In consequence, certain horizons or reflectors of low energy will be disturbed or even destroyed by correlation residues or noises produced by higher-energy horizons located at some distance away.
Finally, it is known that different phenomena have the effect of attenuating the signals more or less selectively as a function of their frequency during emission of said signals, of their path within the medium to be explored and of their reception. Thus, at the time of emission by a vibrator which is capable of producing a number of frequencies, coupling of the vibrator with the surface layer of the medium to be explored or in other words the influence of said surface layer on which said vibrator rests on the emitted frequencies is such that the vibrator which is set at a low frequency such as 6 Hz, for example, emits at 6 Hz only to a limited extent but especially harmonics of this frequency such as 12 or 18 Hz, for example. As a result, this coupling limits the energy of the low-frequency signals and may therefore introduce errors in the corresponding signals received which are liable to be interpreted as being those produced by an emission of one of the harmonics of the low frequency. Moreover, there takes place a selective attenuation of the high frequencies by the medium to be explored as a result either of an effect of inelastic absorption of rocks or of the filtering action of geological series having thin-layered strata. In point of fact, both in regard to emission of low and high frequencies and in regard to reception of reflected waves, the conventional correlation method does not make it possible to dissociate the effects of coupling or of selective attenuation at high frequencies.
In seismic exploration as a whole, the high frequencies are those which are higher than 75 Hz.
The object of the present invention is to overcome the disadvantages mentioned in the foregoing and to propose a novel method of seismic exploration which makes it possible in accordance with the vibrational seismic technique to ensure that all the desired frequencies are received with the highest possible signal-to-noise ratio.